1. Field of the Invention
The present invention relates to an output voltage controller of an engine-driven generator and, in particular, to an output voltage controller of an engine-driven generator that keeps the output voltage of a synchronous generator stable and facilitates the control of temporarily stopping output of power generation when an excessive load is applied or restarting the generator.
Moreover, the present invention relates to an output voltage controller of an engine-driven generator capable of suppressing a self-excited action produced by the relationship between a flywheel diode and a capacitive load, which are connected in parallel to a field winding, to stabilize the output voltage of the generator.
2. Description of the Related Art
FIG. 6 is an electric wiring diagram of an automatic voltage controller which is a conventional output voltage regulator in the related art for suppressing variations in the output voltage of a generator. The automatic voltage regulator shown in FIG. 6 includes an exciting winding 101 of the generator, a field winding 103, and a detection winding 104 for detecting an output voltage. A detection circuit 109 constituted by a full-wave rectifier 105, a capacitor 106, and resistances 107, 108 rectifies the full wave of the detection output of a generator output from the detection winding 104 by a full-wave rectifying circuit 105 and inputs an output voltage Vc detected on the basis of this full-wave rectified waveform to a control circuit 102. The control circuit 102 compares the output voltage Vc with a reference voltage Vz set by a Zener diode. If Vc<V2, the control circuit 102 supplies the output current of the exciting winding 101 as a field current “if” to a field winding 103. This kind of automatic voltage regulator has widely used because the method for detecting an output voltage and the method for controlling a field current are simple. The automatic voltage regulator like this is disclosed in, for example, Japanese. Publication of examined Patent Application No. 3-41038.
A generator having this kind of automatic voltage controller is generally provided with a breaker having a mechanical contact so as to protect the generator when an excessive load is applied that is caused by the short-circuit of the output or the like. The breaker having the mechanical contact has a structure of pulling apart the contact when an excessive current is detected and has high operating reliability. Thus, the breaker has been widely used.
The automatic voltage controller disclosed in the foregoing patent document supplies a control power from the exciting winding, so the automatic voltage regulator cannot perform the control of shutting off a field current temporarily to stop generation output and then of again supplying the field current to restart generation output. This is because when the field current is turned off so as to stop generation output, the output of the exciting winding also is turned off. That is, it is difficult for the automatic voltage regulator to perform, for example, the control of temporarily stopping the generation output and providing a display showing an abnormal state or alarm when a temporary excessive current state is developed. Thus, when the excessive current is developed, the generator needs to be stopped and then to be restarted.
Moreover, to secure the field current to be supplied to the field winding at the time of restart, the construction of utilizing the remaining magnetism of a rotor itself or the construction of mounting a small magnet used for initial excitation on the rotor has been employed. However, in these constructions, the initial excitation is started at an extremely low voltage, so an FET or the like of low loss and small driving power cannot be used as a switching element for controlling a field power. Thus, a bipolar transistor of large loss needs to be used inevitably.
A current value when output is in the state of short-circuit is different depending on the impedance of a generator but usually is about two times a current value at the time of normal operation. A current of this degree may flow temporarily also at the start of an inductive load. Thus, when a breaker is set so as to be tripped immediately when the current value becomes two times the current value at the normal operation in consideration of the current value in the state of short circuit, the breaker may be tripped also when a large current flows temporarily, for example, when an inductive load is started to apply. This presents the problem of impairing usability. Moreover, when the breaker is tripped, there is brought about a state where output is shut off until the breaker is operated for recovery. Thus, it is impossible to start up the inductive load while performing the control of repeating the temporary shut off and return of the breaker.
In such automatic voltage controller, the current to be supplied to the field winding is controlled by a switching operation, and a flywheel diode connected in parallel to the field winding is generally used so as to absorb a surge developed at the time of this switching operation.
By the way, the generator employing such automatic voltage controller is used as a general power source on the site of civil engineering work and out-of-door work, and there are cases where a capacitive load having a capacitor for improving a power factor, for example, a high power factor mercury-vapor lamp is connected to the power source. When such a capacitive load is connected to the generator, an AC voltage developed in the field winding by the effect of a developed armature reaction is rectified by the flywheel diode, thereby working as an exciting current on the field winding. As a result, there are cases where an abnormal voltage beyond the control range of the automatic voltage regulator is developed to have an adverse effect on the constituent components of the circuit.
As measures against this problem, there has been proposed an automatic voltage controller in which in addition to switching elements for controlling a field current by a switching operation, there is provided with a second switching element that is connected in series to the foregoing switching element so as to be parallel to a flywheel diode and is turned on and off in opposite phases (see Japanese Publication of examined Patent Application No. 5-42239). In this automatic voltage controller, even when an AC voltage is developed and an AC current is flown in the field winding by the armature reaction, self excitation can be suppressed by flowing back the AC current as it is by the operation of the flywheel diode and the second switching element.
[Patent Document 1] Japanese Publication of examined Patent Application No. 5-42239
However, in the controller disclosed in Japanese Publication of Examined Patent Application No. 5-42239, the switching element is only disposed in a small bypass path, so there is a case where a sufficient effect cannot be obtained when a power factor is small and a developed exciting current becomes large or depending on the characteristics of the generator. Moreover, there is presented the problem that measures also need to be taken against the failure of the bypass path itself.